<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Funding Free Software", the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
"GNU Free Documentation License".

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development. -->
<!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Define Subst (GNU Compiler Collection (GCC) Internals)</title>

<meta name="description" content="Define Subst (GNU Compiler Collection (GCC) Internals)">
<meta name="keywords" content="Define Subst (GNU Compiler Collection (GCC) Internals)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html" rel="start" title="Top">
<link href="Option-Index.html" rel="index" title="Option Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Machine-Desc.html" rel="up" title="Machine Desc">
<link href="Define-Subst-Example.html" rel="next" title="Define Subst Example">
<link href="Conditional-Execution.html" rel="prev" title="Conditional Execution">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<span id="Define-Subst"></span><div class="header">
<p>
Next: <a href="Constant-Definitions.html" accesskey="n" rel="next">Constant Definitions</a>, Previous: <a href="Conditional-Execution.html" accesskey="p" rel="prev">Conditional Execution</a>, Up: <a href="Machine-Desc.html" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<span id="RTL-Templates-Transformations"></span><h3 class="section">17.21 RTL Templates Transformations</h3>
<span id="index-define_005fsubst"></span>

<p>For some hardware architectures there are common cases when the RTL
templates for the instructions can be derived from the other RTL
templates using simple transformations.  E.g., <samp>i386.md</samp> contains
an RTL template for the ordinary <code>sub</code> instruction&mdash;
<code>*subsi_1</code>, and for the <code>sub</code> instruction with subsequent
zero-extension&mdash;<code>*subsi_1_zext</code>.  Such cases can be easily
implemented by a single meta-template capable of generating a modified
case based on the initial one:
</p>
<span id="index-define_005fsubst-4"></span>
<div class="example">
<pre class="example">(define_subst &quot;<var>name</var>&quot;
  [<var>input-template</var>]
  &quot;<var>condition</var>&quot;
  [<var>output-template</var>])
</pre></div>
<p><var>input-template</var> is a pattern describing the source RTL template,
which will be transformed.
</p>
<p><var>condition</var> is a C expression that is conjunct with the condition
from the input-template to generate a condition to be used in the
output-template.
</p>
<p><var>output-template</var> is a pattern that will be used in the resulting
template.
</p>
<p><code>define_subst</code> mechanism is tightly coupled with the notion of the
subst attribute (see <a href="Subst-Iterators.html">Subst Iterators</a>).  The use of
<code>define_subst</code> is triggered by a reference to a subst attribute in
the transforming RTL template.  This reference initiates duplication of
the source RTL template and substitution of the attributes with their
values.  The source RTL template is left unchanged, while the copy is
transformed by <code>define_subst</code>.  This transformation can fail in the
case when the source RTL template is not matched against the
input-template of the <code>define_subst</code>.  In such case the copy is
deleted.
</p>
<p><code>define_subst</code> can be used only in <code>define_insn</code> and
<code>define_expand</code>, it cannot be used in other expressions (e.g. in
<code>define_insn_and_split</code>).
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Define-Subst-Example.html" accesskey="1">Define Subst Example</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Example of <code>define_subst</code> work.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Define-Subst-Pattern-Matching.html" accesskey="2">Define Subst Pattern Matching</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Process of template comparison.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Define-Subst-Output-Template.html" accesskey="3">Define Subst Output Template</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Generation of output template.
</td></tr>
</table>

<hr>
<div class="header">
<p>
Next: <a href="Constant-Definitions.html" accesskey="n" rel="next">Constant Definitions</a>, Previous: <a href="Conditional-Execution.html" accesskey="p" rel="prev">Conditional Execution</a>, Up: <a href="Machine-Desc.html" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html" title="Index" rel="index">Index</a>]</p>
</div>



</body>
</html>
